The present invention arose from an effort to develop equipment and methods to compensate for differences in thermal expansion conditions encountered across transverse sections of the pressure roller in a roll-fusing assembly.
Roll-fusing assemblies are well-developed with respect to copying machines and printers designed to individually print and handle single sheet print stock. Background discussions and illustrations can be found in U.S. Pat. Nos. 3,884,623, 4,019,024, and 4,594,068, as well as in U.S. Defensive Publication No. T967,010 (published Feb. 7, 1978).
The roll-fusing apparatus provided in an electrophotographic printer typically includes a yieldable pressure roller mounted in rolling opposition to an internally heated fusing roller. The externally driven fusing roller fuses a toner image on print stock as it passes through a rolling nip between the opposed rollers.
Stock feeding difficulties have been encountered when attempting to adapt electrophotographic printer technology to the printing and handling of continuous length print stock, such as paper or labels supplied in roll or bi-fold configurations. While single sheet electrophotographic printers can accommodate print stock of varying widths, speed variations in the roll-fusing apparatus that result from differences in print stock widths and thicknesses are of no substantial consequence because of the relatively short print stock length over which such differences occur. However, even very small decreases in print stock speed (linear velocity) through a roll-fusing assembly will cause the tension of the print stock located upstream from the roll-fusing assembly to slacken, thereby detracting from resulting print quality.
A one percent speed variation, when continuously running print stock at a typical linear speed of 3 inches per second, will result in a progressively changing length increase of 0.03 inches per second in the print stock immediately upstream from the roll-fuser assembly. The resulting bow or "bubble" in the continuous length print stock (see dashed line 14' in FIG. 1) is unacceptable to quality printing results, which require constant linear speed to be imparted to the print stock as it passes through the roll-fusing assembly.
The conventional fusing and pressure rollers provided within a roll-fusing apparatus have cylindrical exterior surfaces of constant diameter across their respective widths. The heating elements included within the fusing roller are typically continuous across the roller widths, being designed for intermittent operation when printing single sheets or materials. Temperatures across the roller surfaces are substantially constant under normal operating conditions encountered in the roll-fusing assembly when running single sheets. The rollers engage one another directly a substantial proportion of their operational time. Variations in heat transfer patterns across the rollers due to differing print stock widths and thicknesses are transient and averaged between the discrete sheets.
When continuous length print stock of differing widths and thickness is fed through the center of the opposing pair of rollers, meaningful differences in the pattern of heat transfer from the fusing roller to the pressure roller can result across the roller widths. These variations can be attributed to changes in the insulating effect of the print stock interposed between the heated fusing roller and the opposed yieldable pressure roller. Over a period of time the end section of the pressure roller, that directly engage the exterior of the fusing roller, will be warmed to a higher operational temperature than its central section, which is separated from the fusing roller by the continuous length print stock. The thermal variations that result across the rollers can affect printing results in ways not encountered when using single length print stock.
It has been found that continuously running narrow print stock through a roll-fusing assembly designed to also handle wider stock, and differences in the thicknesses of print stocks run through the fuser, can result in greater amounts of thermal expansion occurring at the ends of the pressure roller than at its center. This causes the center of the roller to reduce the feeding pressure on the narrow width print stock, which in turn results in a reduced linear speed being imparted to it and causes misregistration in the printer. Since the upstream linear velocity of the print stock is constant, decreases is the linear velocity of the print stock through the roll-fusing assembly will cause the stock to bow or form a "bubble" adjacent to the fusing roller, detracting from printing quality .